The polarised political debate in the U.S. has created a particularly dangerous element in the Republican Party which is becoming increasingly mainstream. The frequent declarations of hate against organisations such as Planned Parenthood, the use of dehumanising language such as 'murder' and 'monster' have led to terrible acts of violence in the past and it was inevitable that it would happen again.
Extremists are known for having very extreme opinions on all sorts of matters. They tend to foster anger and hate. By sharing these opinions and sending strong messages against the groups they dislike, they are encouraging others to buy into these ideas. When more and more people buy into these opinions, there is a greater likelihood that some person will act upon these feelings of hate, and this is how domestic terrorism is born.
You did not define “extremist” and that's extremely important to define.
Extremists = Nazi-based governments (especially the governments of the United States, moreover, the U.S. Congress & ALL governments in California)
So, with the “extremists” properly defined, it’s without a doubt that they are creating a “poisonous environment” that breeds domestic terrorism, and not just in the United States but Worldwide too, and it will continue until the people of this world get their Nazi government's under control, especially the people of the United States but citizens of the United States are dumb-as-pHuck, they have not been properly educated so they can't tell the difference between a Democracy and Nazis so they will defend their tyrannical government with their blood, all the while committing treason against We the People of the United States at the same time as defending their tyrannical government.
Whether or not extremists are domestic or abroad, their terror attacks receive widespread news coverage. Many may not have even considered mass shootings or bombings as something they could do before seeing examples of it through media outlets. The frequency of these attacks makes it seem easier and easier to future perpetrators.
The global interchange of information promotes a vast communication network for any terrorist looking for a reason to escalate their message using violent tactics. The domestic terrorist is empowered by the attention that extremists are given by the media and the poisonous environment spreads like a cancer. Domestic terrorists have more electronic resources available to gather together their devious cohorts and to carry out their plans.